Method of making metal foams by sequential expansion

ABSTRACT

Metal foams are produced by decomposing a blowing agent in a molten metal such that there is an initial and a subsequent expansion due to blowing gas. The sequential expansion can be conducted by more than one addition of blowing agent to the molten metal to be expanded, or by allowing the molten mass to expand, applying agitation to collapse the foam, and allowing it to expand again, or by forming an intermediate product by expanding and cooling, remelting the intermediate, stirring, and foaming again. Zinc-based foams of fine quality are produced. In a preferred embodiment, substantially pure zinc, titanium hydride (0.1 to 1.5 weight per cent based on weight of metal, and a reaction temperature of from about 475* to about 500*C. are employed.

United States Patent 1 Niebylski et al.

[ Feb. 5, 1974 METHOD OF MAKING METAL FOAMS BY SEQUENTIAL EXPANSION [75]Inventors: Leonard M. Niebylski, Birmingham; Chester P. Jarema, Detroit,both of Mich.

[73] Assignee: Ethyl Corporation, Richmond, Va.

[22] Filed: June 21, 1971 [21] Appl. No.: 155,210

Related US. Application Data [63] Continuation-impart of Ser. No.879,515, Nov. 24,

1969, abandoned.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. R.Satterfield Attorney, Agent, or Firm-Donald L. Johnson; Robert A. Linn[5 7] ABSTRACT Metal foams are produced by decomposing a blowing agentin a molten metal such that there is an initial and a subsequentexpansion due to blowing gas. The sequential expansion can be conductedby more than one addition of blowing agent to the molten metal to beexpanded, or by allowing the molten mass to expand, applying agitationto collapse the foam, and allowing it to expand again, or by forming anintermediate product by expanding and cooling, remelting theintermediate, stirring, and foaming again. Zinc-based foams of finequality are produced. In a preferred embodiment, substantially purezinc, titanium hydride (0.1 to 1.5 weightper cent based on weight ofmetal, and a reaction temperature of from about 475 to about 500C. areemployed.

4 Claims, No Drawings METHOD OF MAKING METAL FOAMS BY SEQUENTIALEXPANSION This is a continuation-in-part of application Ser. No.879,515, filed Nov. 24, 1969 now abandoned.

BACKGROUND OF THE INVENTION Foamed metals have been described in theprior art; see for example, U.S Pat. Nos. 2,895,819, 3,300,296, and3,297,431. Such foams are produced by adding a gas-evolving compound toa molten metal and thermally decomposing the compound to evolve blowinggas. The gas forming compound can be a metal hydride such as ZrH or TiHUS. Pat. No. 2,983,597.

It is known that zinc foams can be produced; confer the aforementionedU.S. patents. In those patents, alloys having an appreciable amount ofmetal other than zinc are taught to form foams. This invention providesmeans to foam substantially pure zinc as well as other metals andalloys.

SUMMARY OF THE INVENTION In essence, this invention provides metal foamsand processes for their formation. In an important aspect, thisinvention provides a process for preparation of a metal foam, saidprocess comprising decomposing a blowing agent in a molten metal toproduce blowing gas such that there is an initial and a subsequentevolution of blowing gas from said blowing agent; and subsequentlycooling the expanded mass produced after the final evolution of blowinggas, thereby forming a set cellular product.

Methods for the sequential expansion include more than one addition ofblowing agent to the molten metal to be foamed,

allowing the molten mass to expand by decomposition of blowing gas,causing the foam to collapse and expand again, and

forming a set intermediate product by expanding and cooling, remeltingthe intermediate, and foaming again.

By such techniques, zinc-based metals including substantially pure zincor alloys having 85 per cent zinc or more, can be efficaciouslyexpanded.

DESCRIPTION OF PREFERRED EMBODIMENTS In a preferred embodiment,substantially pure zinc is employed as the metal to be foamed. Zincalloys having up to about weight per cent of alloying metal(s) such asaluminum-zinc and magnesium-zinc alloys can be used. Typical alloyswhich can be employed are ZDC No. 3 (AG4OA), ZDC No. 5 (AC4lA) and AlloyZDC No. 7. Compositions of such alloys are set forth on pages 28-9 ofASARCO, Zinc Die Casting Alloys, American Smelting and Refining CompanyBulletin VI-l. The compositions of those alloys recited on the citedpages are incorporated by reference herein as if fully set forth.Another typical alloy is ILZRO 12; it has the composition aluminum 11-13 percent copper 0.5-1.25 percent magnesium 0.01-0.03 percent zincbalance Also lead-zinc alloys can befoamed according to the process ofthis invention, so long as they are molten at applicable temperatures,such as the ranges discussed below. Moreover, this invention can beextended to lead and lead alloys. The lead may be alloyed with magnesiumor aluminum. Other alloys can be employed as well as other substantiallypure metals.

The temperature employed is not a truly independent variable but isdependent at least in part on the nature of the metal and the nature ofthe blowing agent employed. The temperature employed is an elevatedtemperature sufficient to maintain the metal in a molten state whileexpansion takes place and also sufficient to thermally decompose theblowing agent to form blowing gas at a rate which yields foaming at thedesired speed. The speed of foaming should not be so long as to undulydelay the process; likewise, it should not be too fast for blow-holeformation or other untoward event may occur. Usually it is desirable toemploy a temperature which allows the sum of the plurality of expansionperiods to total from about 1 to about 45 minutes, but shorter andfaster times can be employed. Given a desired metal-blowing agentsystem, a skilled practitioner can determine a desired temperature andsuitable expansion time sum with a minimum of experimentation. Preferredsums are 2-30 minutes and most preferred from about 3 to about 15minutes. Thus, temperatures of 395655C. can be employed and it ispreferred to use temperatures of from about 425 to about 625C. Morepreferable, temperatures of from about 425 to about 525C. and mostpreferably from about 475 to about 500C, are employed.

Such temperatures are conveniently used when conducting the process byutilizing a metal hydride such as magnesium or titanium hydride as theblowing agent. In general, for this invention, hydride blowing agentsare of choice. The titanium hydride of commerce usually has a littleless hydrogen than the stoichiometric amount. Such commercial materialis very satisfactory, as are materials wherein more hydrogen has beenremoved (by treatment at about 400C. for from about 1 to 24 hours). Whenfoaming a zinc-based metal, that is, a metal which has weight per centzinc or more, titanium hydride affords a finer pore structure than MgllZrI-I prepares foams usually of intermediate pore size.

Thus, where coarser zinc-based foams are desired,

tion. However, greater and lesser amounts can be emv ployed if desired.As the ,amount of blowing agent increases (when other parameters areallowed to remain constant) preparation of less dense foams is favored.

When using titanium hydride, a preferred amount is I from about 3 toabout 10 grams per each 1,000 gram portion of zinc-based metal to befoamed.

As taught above, the process of this invention entails more than oneexpansion of a molten mass utilized to form a set cellular product. In apreferred embodiment, two expansions are employed, that is, an initialexpansion is followed by a final expansion. If desired, more than twoexpansions can be carried out. Thus, three, four or more expansions maybe used.

When the process is conducted, all the blowing agent to be employed canbe introduced by one addition. When this expedient is employed, onemethod of achieving a plurality of expansions is to allow the materialto foam, then cause the foam to collapse, and then allow the molten mass(thereby produced) to foam again. Good results are obtained when theinitial expansion increases the mass to be foamed from about two toabout three times the original volume, and the foam is made to collapseto the original volume or thereabouts, say, up to about 1.1 or 1.2 timesthe original volume.

The foam can be made to. collapse by stirring as described above,however, it is not necessary to do so.

One can add all the blowing agent in one addition, allow the molten massto foam, cool to form an intermediate set cellular product, remelt andstir, and allow the mass to foam again, and then cool to form a finalset cellular product. This embodiment can be carried out in a number ofways. For example, a comparatively large amount of blowing agent can beadded, say up to about 5 to about weight per cent based on weight ofmetal to be formed, allow the molten mass thus formed to foam to anextent greater than desired for the final product, cool to form anintermediate, and then remelt and stir, and finally, allow the moltenmass to foam to the extent desired. Alternatively, after the originaladdition of blowing agent, the foaming can be stopped with less or aboutthe same amount of foaming as desired in the final product. Theseproducts can be remelted and refoamed as pointed out above. Moreover,one can make one addition of blowing agent, allow the first expansion totake place to the extent desired, cool and add some or all theintermediate thereby produced to a further amount of molten metal, allowfoaming to occur to the desired extent, and then allow the final productto cool and set. Routine experimentation can determine the extent ofinitial expansion and the amount of non-blown molten metal which willyield a desired product for any given amount of blowing agent andinitial amount of molten metal used to form the intermediate product.

Alternatively, the blowing agent employed may not all be added in oneaddition. In other words, a plurality of additions of blowing agent canbe utilized. Good results are achieved using two blowing agentadditions, but more can be employed if desired. Thus, one can makethree, four or five or even more separate additions of blowing agent tothe molten mass. When using two additions, generally at least 5 weightper cent of the total amount of blowing agent to be employed is added inthe first addition. In some instances, it is desirable to add at least10 percent of the blowing agent. In many instances it is desirable toadd about 50 per cent of the blowing agent in the first addition.Greater or lesser amounts than those discussed above can be added in thefirst addition, as desired.

The blowing agent need not be added to the molten metal at the sametemperature. In fact, in a preferred embodiment of this invention usingzinc-based foams, a portion of the blowing agent is added at atemperature of from about 425 to about 450C. and subsequently theremainder of the blowing agent is added at a temperature of from about475 to about 525C. When this operation sequence is employed, foamingoccurs over a longer period of time and more uniform mixing of theblowing agent in the molten metal can be achieved. These beneficialresults are enhanced by the temperatures of 425 450C., since thatusually affords very slow foaming.

The above embodiments may be combined. For example, one can add blowingagent to a molten mass, allow it to foam to form an intermediate setcellular product, remelt, add a second quantity of blowing agent, allowthe mass to foam to the desired extent, and then cool to form the finalset cellular product.

For the process of this invention to achieve its best results, it isdesirable that the initial expansion occur to a significant amount. Theexact amount of initial expansion required per given embodiment and pergiven amount of blowing agent in molten metal can be determined for eachagent and metal via routine experimentation. Generally, good results areachieved if upon initial expansion the molten mass expands to form abouttwo to three times the initial volume. Greater or lesser amounts ofexpansion can be employed. For example, good results can be achievedwhen the initial expansion occurs to about 1 10 per cent of the initialvolume. Similarly, good results have been achieved when the initialexpansion was about 15 times the original volume and three times thefinal volume. Thus, the final expansion can be to a greater or lesserextent than a previous expansion. When an intermediate set cellularproduct is to be formed and this is to be added to additional moltenmetal (and no more blowing agent is to be added to the final melt) theamount of initial expansion, generally, is equivalent to about 1.1 toabout three times the volume of the final amount of molten metal.

The ability to form satisfactory foams utilizing an initial expansionfollowed by a cooling step to form an intermediate set cellular producthas two advantages. First, it allows scrap foam to be utilized. Second,it allows the formation of set cellular product at one location andshipment to another for final expansion.

In an important aspect, this invention involvesthe discovery that betterresults are achieved if the molten mass to be foamed had been subjectedto an initial prior treatment with blowing gas. The reason for thebeneficial result obtained is not known with certitude. Not wishing tobe bound by any theory, the beneficial result may occur throughincreased wetting of the blowing agent employed. It has been noted thatin certain metals such as zinc, upon initial addition of blowing agentsuch as titanium hydride, that even after rapid, efficient stirring,some metal hydride particles will come to the surface of the moltenmetal. If the molten metal is subjected to an initial addition oftitanium hydride followed by sufficient stirring and some expansion asdescribed above, it has been noted that there is generally muchless-blowing agent on the surface after a second addition of blowingagent. These results tend to hypothesize that pre-gassed metal may havean enhanced ability to wet blowing agent particles such as metalhydrides.

In a preferred embodiment, the temperature of the massto be foamed withTil-I, is maintained by an induction field. For reasons unknown to us,it appears that induction heating affords higher quality foams thanresistance heating methods. In addition, induction heating afi'ordscloser process control.

As appreciated by a skilled practitioner, better foams are produced ifthe blowing agent is substantially'uniformly mixed throughout the moltenmetal to be foamed. Thus, we prefer to use mixing techniques which tendto give efficient mixing. High speed stirring is a method of choice.

Our process proceeds well at ambient pressure, but greater and lowerpressures can be used if desired.

The following exemplary material serves to illustrate the process ofthis invention.

EXAMPLE I In a cylindrical reaction vessel, a 1,200 gram charge ofsubstantially pure zinc was heated to 440C. To this was added four gramsof Til-l enclosed within lead foil. The addition of titanium hydride wascarried out while dispersing, using high speed mixing, for approximately30 seconds. Very slight foaming occurred. An additional 5 gram portionof titanium hydride was added and stirred in for approximately 30seconds while the molten metal was at a temperature of 470C., utilizingan induction field. While this temperature was maintained, foamingcontinued for 4-5 minutes. Upon cooling, a good quality foam of l2l 3per cent density with very small (1/64-1/32 inch) average pore sizecells was produced.

Similar foams are made when the amount of titanium hydride is from 1 tograms per 1,000 gram portion of zinc. Similar foams are also producedwhen the same amounts of magnesium hydride are employed but in generalthey have larger pores than analogous TiH foamed materials. Likewise,analogous foams are produced from zinc alloys having up to about 15weight per cent of alloying material selected from magnesium, aluminum,zinc, and combinations thereof. Similar foams are produced when theprocess of the above invention is continued at temperatures up to about625C. However, the foaming is more rapid and in many instances, greateramounts of blowing agent are preferably employed.

Our work has indicated that zinc foams are made in fine pore qualitiesonly if they'have been subjected to at least two foam operations. Ifonly one expansion is allowed to take place, the resultant foam will belarge celled, non-uniform, andwill usually have a heavy skin bottom.Such inferior foams can be produced for example, using pure zinc and0.75 1.25 grams of titanium hydride per each 100 gram portion of zincemployed.

EXAMPLE II A 1,120 gram portion of substantially pure zinc was meltedand 60 grams of substantially pure aluminum were added to the zinc at450C. The temperature was raised to 600C.

Thereafter, the temperature was brought to 460C. and 10 grams of Til-lwere mixed with the Al-Zn alloy. The temperature dropped to 420C., andin spite of admixing with efficient stirring, some of the hydrideremained on top of the alloy. The resultant mixture was reheated suchthat it reached 480C. In approximately 5 minutes. During this reheating,it foamed slowly.

The foam was restirred such that the melt approached the initial volumeand reheated to 480C. After 4 minutes at this temperature foamingstopped.

The crucible containing the molten mass was removed from the inductionfurnace. It was noted that some foam collapse had occurred. Sectioningrevealed collapsed cells and a k inch thick bottom skin.

EXAMPLE, 111

A 953 gram portion of substantially pure zinc was melted and 43 grams ofsubstantially pure aluminum were added thereto at 450C. The temperatureof the resultant mass was raised to 600C. to insure alloying.

The temperature was subsequently brought to 500C. and solid carbondioxide snow was added to the molten mass until the temperaturedecreased to 400C. At that temperature the metal was too cold to add anymore carbon dioxide snow. A total of l 19 grams of CO had been added.The resultant melt was heated to 600C. and an additional 132 grams of COwere added. This brought the temperature to 485C. and the molten metalwas very thick.

At 470C, 10 grams of titanium hydride were added. The metal appearedcold as many solids had formed. The melt was reheated for 2 minutes andat a temperature of 490C. Foaming occurred until the volume was abouttwice the original volume. The mixture was restirred until the volumeapproached the original; the metal was very heavy and as a result mixingwas difficult. The induction field was applied again, foaming reoccurredand when the foam reached about 2 inches from the top of the cruciblecontaining it, the crucible was removed from the induction furnace.

Upon standing the foam shrunk somewhat. Sectioning revealed a betterfoam quality than that obtained in Example ll. Cell collapse .wasminimal and the bottom was one-eighth inch as compared to the /2 inchskin in Example ll. Thus, carbon dioxide improves the quality ofaluminum-zinc foams. Similar improvements in quality are achieved withwater, nitrogen, and argon. Air or oxygen can also be used to increasethe viscosity of the melt.

EXAMPLE IV A 1,200 gram portion of zinc balls was melted and thetemperature raised to 450C. At that temperature, two lead foil packetsof 5 grams titanium hydride each were added with stirring. The stirrerwas removed and the resultant mass was heated for 5 minutes. Very littlefoaming occurred during that period in which the final temperature was430C. The molten mass was reheated to 450C. and foaming began. Themixture was restirred causing the volume to decrease to about theoriginal volume. Thereafter the stirrer was removed and the resultantmass reheated for 5 minutes at about 480500C. During that period a heavylid had been placed on the crucible containing the foam. During theresultant second foaming operation, the zinc foamed to fill the crucibleand push the lid up. After cooling to obtain a set cellular product,sectioning revealed a very fine pore foam with no appreciable skin.Utilizing this technique such that before the final expansion themixture is allowed to foam to 2-3 times the original volume affords goodresults.

EXAMPLE v After minutes in an oven, it was noted the foam had a purplishcolor; after 8 minutes, the foam was found to have collapsed somewhat.Sectioning after cooling revealed collapsed cells.

A 960 gram portion of the intermediate foam pro- Compressive Strengthsof Various 1" X l" X 1" Zinc Foarn s Strength to Weight Ratio in psi/pcfFoam Description Wt. (pcf) Comp. Strength (psi) Pure Zn w"/TiH 4.2g I6056 3.50 Do. 4.5g 17.1 56 3.27 Do. 4.2g l6.0 55.8 3.49 Pure Zn w/CO andTiH 3.7g [4.] 43.8 3.l2 Do. 5.2g 19.8 60.0 3.03 5% Al 95% Zn w/CO andTiH 9.9g 37.7 328 8.70 Some crumbling Do. 10.6g 40.4 433 10.7 Do. AlloyNo. 5 WITH: ll.0g 41.9 317 7.56 Do. Do. 13.4g 5].[ 698 13.7 Do. 371Mg-97'7c Zn w/TiH 15.2g 57.9 345 5.96 More severe crumbling and breakingDo. 169 64.4 372 I35 Do. 371 M g-97% Zn w/ZrH. 2|.7g 82.7 2000 24.- Do.

w=with duced above was melted and began to foam around 450C. Stirringwas conducted, the stirrer was removed and the induction field appliedto give a temperature of about 480C. At that temperature foamingoccurred again and the induction field was shut down. The resultantmass. was air cooled and sectioning revealed a very fine pore uniformfoam. Density was 48.3 pcf, 1] per cent metal density.

dride will'ordinarily liberate enough gas to foam a metal mass at375400C. and higher, titanium hydride will do so at 435-450C. andhigher, and zirconium hydride at 465485C. and higher. In general, bestresults are obtained in this invention with the above blowing agents byutilizing a molten metal at a.

temperature within from about to about 70C. of the lower temperaturecited above for the selected blowing agent. More preferably, the moltenmetal should be generally about 50C. above the lower temperature.Utilizing this temperature-blowing agent relationship, good results areobtained utilizing magnesium hydride, zirconium hydride or titaniumhydride in processes similar to the above examples employed with manydiverse metals and alloys. Suitable alloys may be binary, ternary orquaternary. Such combinations of lead, zinc, magnesium, tin, silicon,copper and aluminum are useful in this invention. We can use the lowmelting eutectic of 32 per cent Al and 68 per cent magn esium.

Ternary systems applicable for-this invention include combinations ofthe above named metals exemplified by aluminum-magnesium and silicon.Thus, we can use an alloy with about 3 to about 15 per cent silicon and04-10 per cent magnesium, the balance being aluminum. Other ternarysystems which we can use are Al- Cu-Mg, Al-Zn-Mg and the like.Quaternary mixtures include combinations of the above named metals.Typical alloys of this type'are Al-Mg-Cu-Zn such as A1 78.5 per cent, Mg5 per cent, Cu 5 per cent, and 11.5 weight due to the small sample size.Nevertheless, trends can be ascertained. These results indicated thatalthough zinc alloys of aluminum and magnesium are more difficult tofoam than pure zinc, such alloys have a much better strength to weightratio than substantially pure zinc foams. The results also indicate thatcarbon dioxide thickening of pure zinc produces a foam having astrength-weight ratio about the same as a nonthickened pure zinc foam.Also, the data indicate that magnesium or aluminum additions to zinctend to cause foam embrittlement.

The foamed products of this invention can be utilized in many instanceswherein the non-foamed base metals are employed. Foamed zinc can beutilized in structural materials where zinc is now used and less weightis desirable. The zinc foams of this invention can be used in injectionmolding and diecasting complex shaped hardware for automobiles,aircraft, and other vehicles. For structural use, slabs of foamcan bemade by pouring the mixture to be foamed into a mold having a flatsurface. lf the mold surface has a temperature of, say, 50 to C. belowthe temperature of the expanded mass, then a skin of non-cellular metalcan be formed on the foamed mass. Similar conditions hold for lead andlead alloy products of this invention. The lead and lead alloy foams ofthis invention lend themselves well to use as acoustical materials.

We claim:

1. Process for preparing a metal foam, said method comprising a.decomposing a blowing agent in a molten metal.

thereby producing gas which expands said molten metal to about two up toabout threetimes the initial volume of said molten metal,

b. collapsing the foamed mass to about the initial volume of the moltenmetal by agitation with sitrring means,

c. heating to re-expand said molten metal, and

d. cooling the re-expanded mass to form a set cellular product.

2. A process of claim 1 wherein said metal is a zincbased metal havingup to about 15 weight percent of one or more alloying metals.

3. A process of claim 1 wherein substantially pure zinc is employed.

4. Process of claim 1 wherein said blowing agent is selected from theclass consisting of magnesium hydride, zirconium hydride, and titaniumhydride.

UNITED STATESPA'EENT UFFICE" (/-1) m CERTIFICATE F @QREQTI N P acent NO.3,790 I Dated February 5:

Renard M. Niebylski et a1 a in tha ahove-idantif gmd pment i It 15certifiiefi mm: 12mm appaw am?" that mid Lettara mama mm hamby awmcm a3shown balm Invntofls) r Goluiam l, 7 line 21, imam am the follmwingsentence:

-- This is a cant-mmtiondmgar-t m applicatian Serial No. 879,515, filwfiavember 2 1969,

now abandonefi.

Signed and sealed this 15th day of October 1974.

-(SEAL) Attest:

MCCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents

2. A process of claim 1 wherein said metal is a zinc-based metal havingup to about 15 weight percent of one or more alloying metals.
 3. Aprocess of claim 1 wherein substantially pure zinc is employed. 4.Process of claim 1 wherein said blowing agent is selected from the classconsisting of magnesium hydride, zirconium hydride, and titaniumhydride.